自由场典型液化特征数值模拟试验
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摘要
应用FLAC3D实现自由场液化数值模拟试验。试验结果验证了砂土液化典型特征:超静孔隙水压升高,有效应力降低,体积压缩积累增大。证实了液化的隔振作用:砂土在液化状态变为流体,不能传递剪力,液化时砂土位移、速度、加速度振幅显著降低,剪应力降低,动水向上渗流,土体向下沉降,水平残留不可恢复位移。球压应力、有效压应力、动水压力满足有效应力原理,但有效压应力的减小与超静水压力的增加不是等量关系。试验表明,激振频率对液化特性有影响,输入频率接近场地自振频率,场地液化程度高,场地位移、变形大。低频输入场地底部液化程度高,高频输入场地顶部液化程度高。输入加速度小液化程度低,输入加速度大液化程度高。采用Byrne塑性模型作不同相对密度自由场液化试验,密实砂土液化程度低,松散砂土液化程度高,超静孔隙水压比稳定值达0.8。作干砂Finn模型数值试验,结果表明,干砂自身的体积压应变比饱和砂大,有效压应力等于球压应力且逐渐减小,干砂没有隔振性能。研究成果为液化场地下结构动力分析提供理论及试验基础。
The numerical simulation test was completed for the liquefaction in free field with FLAC3D.Experimental results show the typical characteristics of soil liquefaction: excess static pore-water pressure increases,effective stress decreases,and volumetric strain increment increases.The isolation effect of liquefaction is confirmed: the sand changes into fluid in the liquid state,can not transfer shear,the displacement,velocity and acceleration amplitudes of sand in liquefaction reduce significantly,shear stress decreases,dynamic water flows upward,soil settlement occurs.The unrecoverable horizontal displacement of soil remains.The ball pressure stress,effective stress,and pore pressure follow the principle of effective stress,however,the decrease of the effective stress is not equal to the increases of the excess pore pressure.The results show that,the excitation frequency influences the liquefaction characteristics,when the input frequency is close to the natural frequency of the site,the degree of liquefaction is high,and displacement and deformation is large.The degree of liquefaction at the bottom of field in low-frequency input field is higher,the degree of liquefaction at the top of field is high in high-frequency input field.The degree of liquefaction is low with smaller acceleration input,the degree of liquefaction is high with larger acceleration input.We completed liquefaction test on Byrne plastic model with different relative densities for free field.The degree of liquefaction is high for low density soil and the degree of liquefaction is low for high density soil.The excess pore water pressure ratio stable value tends to 0.8.We completed numerical test on Finn model for dry sand.The results show that the volumetric strain increment of dry sand is larger than saturated sand,the effective stress is equal to ball compressive stress and decreases and the dry sand does not have isolation performance.Researching results will provide a theoretical and experimental basis for the dynamic analysis of underground structures in liquefied soil layer.
The numerical simulation test was completed for the liquefaction in free field with FLAC3D.Experimental results show the typical characteristics of soil liquefaction: excess static pore-water pressure increases,effective stress decreases,and volumetric strain increment increases.The isolation effect of liquefaction is confirmed: the sand changes into fluid in the liquid state,can not transfer shear,the displacement,velocity and acceleration amplitudes of sand in liquefaction reduce significantly,shear stress decreases,dynamic water flows upward,soil settlement occurs.The unrecoverable horizontal displacement of soil remains.The ball pressure stress,effective stress,and pore pressure follow the principle of effective stress,however,the decrease of the effective stress is not equal to the increases of the excess pore pressure.The results show that,the excitation frequency influences the liquefaction characteristics,when the input frequency is close to the natural frequency of the site,the degree of liquefaction is high,and displacement and deformation is large.The degree of liquefaction at the bottom of field in low-frequency input field is higher,the degree of liquefaction at the top of field is high in high-frequency input field.The degree of liquefaction is low with smaller acceleration input,the degree of liquefaction is high with larger acceleration input.We completed liquefaction test on Byrne plastic model with different relative densities for free field.The degree of liquefaction is high for low density soil and the degree of liquefaction is low for high density soil.The excess pore water pressure ratio stable value tends to 0.8.We completed numerical test on Finn model for dry sand.The results show that the volumetric strain increment of dry sand is larger than saturated sand,the effective stress is equal to ball compressive stress and decreases and the dry sand does not have isolation performance.Researching results will provide a theoretical and experimental basis for the dynamic analysis of underground structures in liquefied soil layer.
引文
[1]Geoffrey R Martin,LiamFinn WD,Seed HB.Fundamentals of liquefaction under cyclic loading[J].Journal of the geotechnical engineering di-vision,MAY1975,423-438.
[2]Seed HB,Sliver ML.Settlement of dry sands during earthquakes[J].Journal of Soil Mechanics and Foundation,ASCE,1972,(4):381-397.
[3]Peter M B.A cyclic shear-volume coupling and pore pressure model for sand[C]∥Second International Conference on Recent Advances inGeotechnical Earthquake Engineering and Soil Dynamics.March 11-15,1991,St.Louis,Missouri,Paper No.1.24,47-55.
[4]陈育民,徐鼎平.FLAC/FLAC3D基础与工程实例[M].北京:中国水利水电出版社,2009:298-299.
[5]郑永来,杨德林,李文艺,等.地下结构抗震[M].上海:同济大学出版社,2007:101-103.
[6]孙锐,袁晓铭.地震荷载下饱和砂土孔压增长时程计算方法[J].地震工程与工程振动,2005,26(3):261-263.
[7]孙锐,袁晓铭,李雨润,等.循环荷载下液化对土层水平往返变形的影响[J].西北地震学报,2009,31(1):8-14.
[8]宫必宁,赵大鹏.地下结构与土动力相互作用试验研究[J].地下空间,2002,22(4):320-324.
[9]陈国兴.岩土地震工程学[M].北京:科学出版社,2007:296-324.
[2]Seed HB,Sliver ML.Settlement of dry sands during earthquakes[J].Journal of Soil Mechanics and Foundation,ASCE,1972,(4):381-397.
[3]Peter M B.A cyclic shear-volume coupling and pore pressure model for sand[C]∥Second International Conference on Recent Advances inGeotechnical Earthquake Engineering and Soil Dynamics.March 11-15,1991,St.Louis,Missouri,Paper No.1.24,47-55.
[4]陈育民,徐鼎平.FLAC/FLAC3D基础与工程实例[M].北京:中国水利水电出版社,2009:298-299.
[5]郑永来,杨德林,李文艺,等.地下结构抗震[M].上海:同济大学出版社,2007:101-103.
[6]孙锐,袁晓铭.地震荷载下饱和砂土孔压增长时程计算方法[J].地震工程与工程振动,2005,26(3):261-263.
[7]孙锐,袁晓铭,李雨润,等.循环荷载下液化对土层水平往返变形的影响[J].西北地震学报,2009,31(1):8-14.
[8]宫必宁,赵大鹏.地下结构与土动力相互作用试验研究[J].地下空间,2002,22(4):320-324.
[9]陈国兴.岩土地震工程学[M].北京:科学出版社,2007:296-324.
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